Nickel metal hydride batteries used for DC power supplies are characterized in that they have high energy densities and long battery lives and are environment-friendly as compared with lead-acid batteries. In addition, the nickel metal hydride batteries are compact and lightweight and have good portability, and hence have recently and rapidly become widespread as on-vehicle batteries and power supplies for countermeasures against disaster.
In order to cope with a recent abrupt increase in power demand for communication equipment, it is necessary to form a power supply system (battery system) having a large capacity, e.g., an output of 30 kWh, by connecting battery sets such as nickel metal hydride batteries in parallel with each other so as to increase the capacity of a DC power supply. In general, when nickel metal hydride batteries are to be used as a power supply system, in order to increase the capacity and service life of the power supply system, a large-capacity nickel metal hydride battery system is formed by connecting k single nickel metal hydride batteries (with an average voltage of 1.2 V and a current capacity of 95 Ah) called single cells in series with each other to form one unit (to be referred to as a module hereinafter), connecting m modules in series with each other to form an assembled battery, and connecting n assembled batteries in parallel with each other.
Various proposals have been made for mechanisms for managing the supply capacity of power for a load in a large-capacity battery system. For example, Japanese Patent Laid-Open Nos. 2004-119112, 2004-120856, and 2004-120857 disclose management methods in power supply systems each including a plurality of assembled batteries connected in parallel with each other, a charge control means, and a discharge control means.
Japanese Patent Laid-Open No. 2004-119112 discloses a technique of internally storing the manufacturing date of an assembled battery, calculating an available period during which predetermined power can be supplied, on the basis of the stored manufacturing date of the assembled battery, and displaying an assembled battery replacement date, in order to facilitate estimation of the service life of the assembled battery at the time of maintenance inspection.
Japanese Patent Laid-Open No. 2004-120856 discloses a technique of providing a battery monitoring means for executing a discharge capacity test on a given assembled battery as a deterioration determination target upon fully charging assembled batteries other than the deterioration determination target as well as the target assembled battery in order to allow power supply to the load side even if a power failure or the like occurs during the execution of the discharge capacity test for deterioration determination of the assembled battery.
In addition, Japanese Patent Laid-Open No. 2004-120857 discloses a technique of providing, for the purpose of reducing a power cost by leveling power demands, a battery monitoring means for monitoring whether the residual capacity of an assembled battery has become equal to or less than a charge start threshold, and if the residual capacity has become equal to or less than the charge start threshold, starting auxiliary charging of the assembled battery at a predetermined time late at night when power is used relatively less.